Currently many metals are recovered by hydrometallurgy methods. These often consist of combinations of leaching, adsorption and electro-winning or precipitation. However, disadvantages exist with each of these processes from the view of economics of the process, purity of the metal obtained, toxicity of the materials used, difficulty of recovery of the pure metal, and the like. For example, conventional processes for mining gold involve extracting the gold from the ore using a basic cyanide solution. The dilute solution is purified and concentrated by various ways, normally solid adsorbents. Techniques to recover other metals are as troublesome and expensive. In particular, the presence of other undesired metals together with the valued ones decrease the efficiency of the process.
W. I. Harris, et al. in "The Extraction of Gold from Cyanide Solutions Using Moderate Base Polyamine Ion Exchange Resins," Reactive Polymers, Vol. 17,1992, pp. 21-27 describe that two polyamine moderate base resins were synthesized from 1,3-diaminopropane or 2,4-diamino-2-methylpentane and chloromethylated styrene-divinylbenzene copolymers. The resins were evaluated for gold recovery from cyanide solutions. The effects of solution pH on the resin gold loadings were determined over the pH range 5-12. The selectivities of the resins for gold cyanide over base metal cyanides were determined. These moderate base polyamine resins were compared against dimethylamine type resins with high and low salt splitting capacity for gold cyanide recovery. The resins possess good selectivities for gold, good loading capacities, good elution properties and can operate at the pH of most leaching streams without acid adjustment.
Large organic ring structures are known to be useful as chelating agents for selective binding and extraction of cartons. Cyclic organic carbonates have been found, along with mixtures of the same with other organic liquids that are miscible therewith, to extract certain metals in stable complex or simple salt form from a medium in which the solvent is not completely soluble, according to U.S. Pat. No. 3,912,801. Extractable metals include gold, platinum, palladium, rhodium, iridium, cobalt, copper, vanadium, uranium, bismuth, cadmium, mercury and cerium. However, the method of this patent is limited in a number of ways. First, the use of an organic solvent that is (a) miscible with the carbonate and (2) is from only slightly soluble to insoluble in the medium is so preferred in this teaching as to be essentially required. Solvents contended to be suitable include benzene, o-xylene, m-xylene, p-xylene, diethyl ether, 4-methyl-2-pentanone, 2,4-pentandione, ethyl acetate, n-heptyl alcohol and chloroform. It should be noted that almost all of these allegedly "suitable" solvents are in essence highly unsuitable for one or more reasons. For example, many have toxicity concerns such as being carcinogens, e.g. benzene. Other materials are too volatile, have strong odors, flash points which are too low or are dangerous due to peroxide formation. For example, diethyl ether is very flammable and has a severe fire and explosion hazard. Chloroform is a known carcinogen and further is a hazard to the ozone layer. Second, the only cyclic carbonates demonstrated as effective are ethylene carbonate and propylene carbonate. Third, to achieve immiscibility or improved extraction, the '801 patent frequently uses added NaCl--this technique is poor because salt is highly corrosive.
It would be desirable if new materials could be developed which efficiently extracted precious metal ions from aqueous solutions selectively and in an extremely efficient manner. Preferably such a process would not require a solvent other than the carbonate itself. Therefore, the use of toxic, flammable co-solvents could be avoided. Note that the method of the '801 patent must use a solvent with ethylene carbonate because ethylene carbonate is 100% water soluble. In any economical sense, the '801 method is undesirable because of the solubility of the carbonates taught therein in water. Propylene carbonate is soluble in water at 25 g of propylene carbonate to 100 g H.sub.2 O. While NaCl may be added to prevent this, the salt undesirably increases corrosiveness. Either alternative of losing the carbonate to the water or using the salt is disfavored.